CHICAGO (CBS) — Adelor the African lion, one of the most popular animals at the Lincoln Park Zoo, has passed away.

As WBBM Newsradio’s John Cody reports, Adelor had been roaring at the zoo since 1995.

LISTEN: WBBM Newsradio’s John Cody reports

In a recent interview, zoo director Kevin Bell had to speak over Adelor’s roar.

“Adelor is right on schedule. Whenever we do a presentation at the lion house, Adelor immediately starts up as soon as he hears the microphone,” Bell said.

Adelor’s death leaves 15-year-old lioness Myra as the only lion at the zoo.

The zoo’s animal care staff made the “difficult” decision to euthanize the geriatric lion due to progressively deteriorating health and quality of life, a zoo release said.

Zoological manager Mark Kamhout called Adelor one of the zoo’s most popular residents.

“Guests of all ages would flock to the lion exhibit when Adelor began roaring as he announced his territory several times a day,” he said in the release. “He was a wonderful leader of the pride and was very protective and affectionate of the females under his care.”

Adelor was probably most commonly known for “lying on the top of the large boulders of the exhibit where he could survey his territory and monitor his pride,” Kamhout said.

Eighteen years is a long life for a lion, many of whom do not survive their first year of life, the release said. Median life expectancy for lions is about 14 years, and the longest known to have survived in captivity is 26 years, the release said.

Zoo officials are working with the Association of Zoos and Aquariums’ African Lion Species Survival Plan to try to find another lion or lions to accompany Myra, the release said, but nothing has as yet been determined.

A memorial page for Adelor fans to comment has been set up at www.lpzoo.org/adelor.

The Sun-Times Media Wire contributed to this report.